Set Appliance State:
Gas On; Pilot On
Set Multimeter to mVolts DC 
Isolate Mains Power to appliance as per TOPs P4
Measurement:
Ignite pilot by depressing the gas control knob and pressing the ignition button.
Keeping the gas control knob depressed, check for voltage on each of the high limit thermostat terminals.
Both readings should be 8 millivolts or above.
Keeping the gas control knob depressed, check for voltage on each of the high limit thermostat terminals.
Both readings should be 8 millivolts or above.
Isolate as per TOPs P4
and refit as necessary